Complex-value signal processing is usually used in modern transmitting and receiving arrangements (for example in mobile radio) which have a relatively wide bandwidth. In this case, quadrature modulators, for example vector modulators, are used for frequency conversion in the transmission direction. In the reception direction, for example in a low IF receiver structure, a local oscillator signal (LO signal) is used to down-mix the received signal to a relatively low intermediate frequency level. The requirement that the local oscillator signal and the carrier signal must be in the form of complex signals, that is to say comprise two signal components which have been phase-shifted through 90° with respect to one another, is imposed on transmitter and receiver architectures having complex-valued signal processing. An orthogonal signal of this type comprises an in-phase signal component and a quadrature signal component.
However, conventional local oscillators which are usually used to produce carrier signals and local oscillator signals provide only so-called single-ended signals. Phase shifters are usually used to produce the phase shift which is therefore still required to produce two orthogonal signal components, with the accuracy of the phase shift directly influencing the receiver's characteristic data, in particular as regards mirror-image frequency suppression.
In addition, the oscillator in modern transmitting and receiving architectures does not usually oscillate at the transmission frequency but rather at a frequency which can be converted to the transmission frequency with as little effort as possible. This is associated with the advantage that effects of the transmission frequency on the oscillator, which is in the form, for example, of a VCO (Voltage Controlled Oscillator), can be diminished.